ROUNDUP, Mont. (AP) -- Another Montana town Thursday was swamped with floodwaters that have washed out roads and rushed though houses across the state, and hundreds more homes downstream in the Dakotas could be hit as heavy rains and melting snow force record releases from bloated dams on the Missouri River.

Ongoing flooding in beleaguered Montana could end up being the worst in decades, officials warned.

The conditions are ripe: unusually heavy snowpack in the mountains, persistent spring rains and waterlogged ground incapable of soaking up any more moisture.

The U.S. Geological Survey said Thursday it was measuring record flows in many rivers and streams, topped by the larger Yellowstone and Musselshell rivers.

"These are major floods, and we know that people's lives could potentially be at risk," said John Kilpatrick, the agency's Montana water science center director.

The U.S. Army Corps of Engineers predicted hundreds of homes in downstream states could flood as the water flows eastward toward North Dakota. The problem could persist into July in the Dakotas, Iowa, Nebraska and elsewhere.

The agency was being forced to plan for record releases of water to relieve reservoirs nearing the top of the six big dams that contain the Missouri River, a river that ultimately collects much of the water in the region.

In Bismarck, N.D., residents were being told to prepare to leave their homes if necessary as the Army Corps of Engineers readied to dump more water than predicted from the Garrison Dam. Animals at the Dakota Zoo were being evacuated Thursday to other zoos in the Dakotas, and the National Guard was beefing up its presence with hundreds more soldiers.

South Dakota also braced for floods. Officials warned that 100 to 150 homes in Pierre and another 200 in Fort Pierre would be hit by flood waters or underground water seeping through basement floors. Residents were moving furniture and other possessions to safe havens and trying to protect homes with sandbags.

"It's unreal," said Lahnee Martin, 22, who was piling sandbags around the Fort Pierre house she just bought a year ago. "It's hard to believe it's going to happen."

In Montana, more roads and highways were being closed Thursday and three more counties declared flood emergencies as dozens of rivers and streams overflowed their banks. Interstate 90 east of Bozeman was reduced to one lane of travel, and only one route to the central Montana town of Roundup remained open.

Roundup was the latest victim of the ongoing flooding, as up to 6 feet of water coursed through the mining and agricultural town of about 1,800 and forced the evacuation of roughly three dozen homes. State officials were sending cots and bottled water to help.

"There's never been water like this since we've been here," said 88-year-old Dan McCaffree, a retired mechanic and rancher.

A dike along the river's north bank gave way in the middle of the night, residents said, sending the river surging through the southern part of the town. The dike was built following a 1967 flood but couldn't handle a river bursting with 13 times more water than normally seen this time of year.

Oil drums, gas cans, a telephone pole and trees were in the water floating through residential areas and swamping vehicles.

The river was expected to crest between Friday and Sunday, authorities said. Thirty to 35 homes were evacuated.

More rain was forecast through the weekend as flood emergencies were announced in Jefferson, Sanders and Musselshell counties. Musselshell County residents were urged to conserve water Thursday morning until emergency services could get an update on the situation.

Relief officials said drinking water, diapers and formula for infants will be distributed over the next two days on the Crow Reservation, where dozens of families were left homeless by earlier flooding from the Little Bighorn River. Floodwaters have retreated on the reservation, but problems persist.

Albert Richmond, a National Weather Service meteorologist, says another "fairly big storm" is expected to move through the area Sunday night into Monday, bringing 2 to 4 inches of rain.

"It's not going to help," he said. "A lot of the rivers and creeks will be going down Saturday and Sunday, but they can come back pretty quick."

On Thursday, residents and officials in Carbon, Yellowstone and Big Horn counties continued to pump water from hundreds of flooded basements. The Office of Public Assistance in Yellowstone County was closed for at least two days after it flooded.

By Thursday afternoon, about 300 people were staying at a Red Cross Shelter set up in a residence hall at the Montana State University-Billings campus.

University spokesman Dan Carter said most of the evacuees -- ranging from infants to the elderly -- came from the Crow Reservation.

Along the Yellowstone River, Pompey's Pillar National Monument east of Billings was closed due to flood concerns. It was expected to reopen Friday.

In the small town of Joliet, southwest of Billings, authorities said they have been unable to break up debris trapped beneath a bridge along Rock Creek that has contributed to flooding. An estimated 160 homes had their basements or lower levels flooded when the creek rushed through town Wednesday.

"They have to wait to get that debris out until the water goes down," said Joliet volunteer fire chief Melvin Hoferer. "If we get some heavy rains, it would be a lot longer. It might be three or four more days."

In central Montana, the Petroleum County drinking water system was being threatened by rising waters.

At least 17 county roads were closed in Judith Basin County and many more in Petroleum County.

In western Montana, the National Weather Service reports water levels have reached flood stage in southeastern Missoula and Granite County. The service predicts water levels will rise above flood stage by Thursday afternoon and could continue to swell.